The method to produce a high molecular weight latex resin powder from a high molecular weight latex prepared by the conventional emulsion polymerization is as follows: First, an aqueous solution of a coagulant such as an inorganic salt or acid is added to the high molecular weight latex, then the electrostatic stability of the high molecular weight latex is broken by an emulsifying agent, resulting in the coagulation of high molecular particles in the latex. The coagulated high molecular particles form a slurry and this phenomenon is called coagulation. The slurry formed by the coagulation is simply a kind of multi-particle assemblage which is not formed by a physical bond, so it is easily broken by any foreign shear. Therefore, aging is required to increase the strength of the slurry by the formation of a physical bond. This procedure progresses at high temperature and interpenetration between chains makes the slurry a particle with better strength.
However, the above conventional method requires an excessive amount of coagulant, which makes the coagulation process very fast, so that the coagulation of the particles will be chaotic. This suggests that the final particles will be irregular and the particle size distribution will be wide, leaving questions about the processing. For example, if a powder with a diameter of 450 μm is mass-produced, transportation and storage thereof will be difficult. If a powder with a diameter of less than 70 μm is mass-produced, dehydration and drying will be difficult and in particular scattering of the micro-powder during packing results in loss of the resin, difficulty in powder transportation from line to line, environmental pollution by dust generation and making the working conditions worse.
As an attempt to overcome the above problems, U.S. Pat. No. 4,897,462 introduces a slow coagulation method based on processing under the critical coagulation concentration. However, this method could not overcome the problem of high viscosity in the early stage, which is the typical problem of slow coagulation, so it can only be applied to batch coagulation and not to continuous coagulation. In addition, according to the slow coagulation process, the moisture content in a round-shaped powder is high, suggesting that dehydration is not satisfactory and a powder with an incomplete structure might be produced because of irregular melting of micro-powders by high temperature during aging. The apparent specific gravity of the powder might also be lowered.
According to EU Patent No. 0,611,788, a continuous coagulation process is applied to the slow coagulation. However, the solid content of the slurry is regulated to be low to overcome the problem of high viscosity in the early stage and to secure a regular particle size distribution. The low solid content in the slurry has an advantage of a regular particle size distribution but has the problems of high energy costs for production and a huge amount of waste water.
To overcome the above problems of the conventional methods, it is an object of the present invention to provide a method for preparing a high molecular weight latex resin powder having an excellent caking property, thermo-stability, apparent specific gravity and regular particle size distribution even with a high solid content, compared with the conventional slow coagulation process, and is able to minimize energy costs by circulation of the working solution and reduce waste water.
The above object and other objects of the present invention can be achieved by the following exemplary embodiments of the present invention.